The overall objectives of this study are (1) to increase the DOPASCANtm safety database on healthy controls and subjects with Parkinson's disease and to expand it to include subjects with Essential Tremor and Progressive Supranuclear Palsy and (2) to determine in a nulti-center setting the SPECT outcome measures that optimize the sensitivity and specificity of DOPASCANth with SPECT scanning in the diagnosis of Parkinson's disease and related disorders. A secondary objective of this study is to determine the reproducability of the SPECT analysis in a multi-center setting in sites with varied imaging equipment.